In various biological analyses, assay methods for detecting a target substance using a specific detectable label have been developed so far. For example, in the nucleic acid detection method by hybridization using the labelled nucleic acid, a nucleic acid (DNA or RNA) to be used as a probe is labelled and brought into contact with a sample containing a nucleic acid to be detected under conditions that hybrid can be formed. If the sample contains a nucleic acid having a base sequence complementary to that of the nucleic acid used as a probe, this nucleic acid binds (hybridize) to the probe to form a nucleic acid-nucleic acid hybrid. The target nucleic acid can be detected by measuring the label contained in the hybrid. In the immunoassay using labelled antigen or a labelled antibody, when an antigen is to be detected, it can be detected by labeling an antibody which is specifically bound to the antigen, effecting formation of an antigen-antibody complex, and detecting the label contained in the complex.
As the label to be used in such nucleic acid detection methods and immunoassay, radioactive substances, non-radioactive substances such as biotin or digoxigenin compounds, fluorescent substances, chemiluminescent substances, and the like are exemplified. Among these, chemiluminescent substances have been expected to be useful as the label because of its high detection sensitivity.
The known method of labeling using a chemiluminescent substance is, for example, a method which comprises reacting a nucleic acid bound to an amino linker with a chemiluminescent substance having a group reactive with an amino group to label the nucleic acid with the chemiluminescent substance (Clinical Chemistry, Vol. 35, No. 8, p.1588-1594, 1989).
However, this method requires to prepare an amino linker-bound nucleic acid, which makes the operation complicated. Further, the method has a disadvantage in that a nucleic acid derived from nature cannot be labelled by directly binding it to a chemiluminescent substance.